The manufacturing of bodies of automobiles involves the making of a plurality of openings or holes which eventually have to be closed by closure plugs. The closure plugs must be of a design such that they provide for a sealing effect so that foreign matters and humidity cannot pass through the hole from one side of the metal sheet to the other side. Humidity enclosed within the car body may result in corrosion. Since such holes form also so-called acoustic bridges, the closure plugs are to be designed such as to provide for acoustic dampening.
A great number of various closure plugs have become known. The closure plugs generally comprise a skirt-like shaft and a flange which sealingly engages the adjacent surface of the structural member. The shaft has at its outer surface, below the flange a circumferentially continous or discontinous undercut defined by a shoulder which engages the opposite side of the structural member. In this manner the closure plug is securely retained within the hole. Furthermore, the distance between the shoulder and the flange is dimensioned such that the flange is deflected outwards and upwards when the shaft is inserted into the hole, thereby generating a corresponding force for urging the flange against the structural member. Furthermore, it has become known to design the closure plug in the area of the flange such that deflection of the flange at the same time generates a radial force for urging a shaft portion against the wall of the hole.
These closure plugs of the prior art have proven to be satisfactory as to their function. With respect to holes of sufficient diameters these closure plugs may be readily, also manually, inserted into the holes. If, however, the closure plugs are to be used for holes having a diameter of 10 mm or less, the closure plugs of the prior art can be assembled only under extremely high forces or possibly not at all.